Several kinds of evidence suggest that there are factors (genes, activators, supressors?) other than simply the level of rDNA redundancy, which are involved in the regulation of nucleolar formation. In this proposal, several experiments are designed to examine the contribution that rDNA (particularly the level of redundancy of ribosomal genes) makes to nucleolar function. By more precisely defining the nature of the rDNA contribution we hope to learn what components of nucleolar formation and function are determined by other factors. One set of experiments is designed to establish the range of variability of rDNA redundancy in several different species. From this information we hope to determine if "minimal" genome sizes are selected for or "optimal" sizes are maintained. From this we want to determine why a certain level of redundancy is present. This involves considerations of the relationship of rDNA amount to cell and total genome size (studied in amphibians), rDNA amount in species with especially small genomes (yeast) and rDNA amounts and nucleolar size (examined in nucleolar mutants of axolotl and in split NOR translocation stocks of maize). Two other aspects of the program develop out of this one. One involves an examination of the behavior of the rDNA locus in inheritance; i.e., is this simple mendelian transmission or is transmission of redundancy level affected by factors such as the homologous NOR locus in the same cell of heterozygotes (examined in inbred lines of axolotl and maize)? The other involves a study of the behavior of the rDNA in amplification in oogenesis i.e., do nucleolar mutants and/or inbred lines of axolotl with different rDNA amounts amplify the genes in the same way and to the same extent? In summary, several experiments are proposed to thoroughly examine the rDNA of axolotl and maize in order to gain further insight into the complex factors regulating nucleolar function.